1. Field of the Invention
An object of the present invention is to provide an automatic trigger circuit, notably a resetting circuit that can be used in any electronic circuit. The invention is designed to be used particularly in the field of chip cards. However, it can be used in any field where the standards related to electrical supplies are likely to change because of technological improvements in electronic circuits.
2. Description of the Prior Art
Most of the integrated circuits used at present work with a supply voltage in the region of 5 volts plus or minus 10%. This is also true of circuits fop chip cards. However, in every field there is now a marked trend towards reducing this supply voltage, for example to about three volts or even less. It is stipulated, notably, that all circuits that can be used in portable microcomputers must be capable of operating at 3.3 volts. In the field of chip cards, the circuits should be capable of working at 5 volts as well as 3 volts or even at a lower voltage. This is because chip cards must be capable of being used by equipment supplied with power at 5 volts as well as by equipment supplied with power at 3 volts.
For all electronic circuits, notably for circuits used in chip cards, a factor of security needs to be taken into consideration. Indeed, a chip card must work accurately not only within a specified range of supply voltage but also outside this range in order to prevent any fraudulent individual for example from taking advantage of a malfunction outside this range to discover the secrets of the card and/or modify its memorized contents, for example by adding monetary units. To obtain protection against such attempts, most integrated circuits are fitted out with trigger circuits that inhibit the operation of the card beyond a certain range of voltage.
For example, for a circuit working at five volts, the bottom triggering voltage is located in the region of 3.5 volts and the top triggering voltage is in the region of 6.5 volts. These substantial margins, which correspond to values of more than +/- 10%, are maintained for two reasons:
these triggering thresholds depend on the manufacturing parameters of the integrated circuit and hence vary from production batch to production batch;
these thresholds are often depend greatly on the temperature: hence, to meet the specification of operation at 5 volts +/- 10%, a substantial margin must be applied.
The problem arises even more acutely when the voltage has to be lowered. For example to have an electronic circuit that is functional between 2.5 volts and 6 volts, it would be necessary to choose triggering thresholds of about two volts and seven volts. However, with the temperature and with the changes in manufacturing parameters, these thresholds may themselves vary between 1.5 and 2.5 volts, and between 6 volts and 8 volts.
For high voltages, the problem is less important because, in general, an electronic integrated circuit, or a chip works, well as at six volts as at eight volts. On the contrary, for the low voltages, the circuits do not work well at such as, for example 1.5 volts, and it is then possible to indulge in fraudulent behavior by using certain components and taking advantage of their malfunctioning at low voltage. This problem makes it very difficult to design a circuit for chip cards that is reliable and proof against fraud for supply voltages of less than three volts.
The aim of the invention, therefore, is to provide a trigger circuit that is stable in terms of threshold voltage and can be adjusted, for example, at the start of the life of the electronic component, notably duping a test on this component. This would firstly enable the elimination of the dependence of the triggering voltage on the parameters of the manufactured batch, the temperature and the supply voltage and secondly make it possible to have an adjustable triggering voltage.
The invention overcomes the above-mentioned drawbacks by proposing the use of a differential amplifier having at least one of its branches cascade-mounted with a cell comprising an EEPROM type electrically erasable and programmable floating-gate transistor. The idea lies in the storage, in the floating gate of a transistor of an EEPROM cell such as this, of the electrical charges corresponding to a reference voltage which will be used to determine the triggering threshold of the circuit. Thus, this voltage can easily be set between 0 and 5 volts with high precision. Since the triggering threshold is adjusted during a test on the circuit at the time of its manufacture, dependence on the parameters of the manufacturing method is completely eliminated. The independence of the threshold with respect to variations between production batches and with respect to variations in temperature and supply voltage is obtained by the differential circuit. Another cell of the same type is mounted in the other arm of this circuit. In practice, the difference between the triggering thresholds is compared with a voltage to be monitored.